The oxygen enrichment of combustion air is widely practiced in the operation of industrial furnaces and kilns in order to improve the operation of such combustion equipment. The benefits of oxygen enrichment are well-known and are associated chiefly with the reduction of nitrogen in the oxidant stream. Because oxygen enrichment increases flame temperatures and reduces the amount of nitrogen in the combustion products, benefits are realized in decreased fuel consumption, increased firing rate and furnace throughput, and reduced emissions of particulates and certain other contaminants due to lower flue gas volumes. The beneficial effects of oxygen enrichment can be offset, however, by increased amounts of nitrogen oxides (NO.sub.x) in the combustion products which occur at increased levels of enrichment. These increased levels occur because the thermodynamics and kinetics of the NO.sub.x formation reactions favor increased NO.sub.x yields at the higher temperatures caused by oxygen enrichment. The increase in NO.sub.x formation during oxygen-enriched combustion is described in a paper entitled "Nitric Oxide Measurements in Oxygen-Enriched Air-Natural Gas Combustion Systems" by C. E. Baukal, Jr. and A. I. Dalton in the Proceedings of the Fossil Fuel Combustion Symposium 1990, ASME, PD-Vol. 30, pp. 75-79.
In the present application, the term NO.sub.x includes all oxides of nitrogen formed in the combustion process. The major portion of the NO.sub.x is NO, with a smaller portion being NO.sub.x. Very small amounts of other nitrogen oxides are formed at the combustion conditions described herein.
The reduction of NO.sub.X formation in conventional burners which use air as the source of oxygen to support combustion can be realized by staging the combustion reactions in order to reduce flame temperatures. This is accomplished by partially combusting a portion of the fuel with a sub-stoichiometric amount of primary air and then adding the required amount of additional air as secondary air, and optionally tertiary air, to complete the combustion of the fuel. In a pre-mix burner, fuel and air are initially mixed and combusted in a burner cavity which opens into an enclosed combustion chamber such as a furnace or kiln. The reduction of NO.sub.x in such burners by staged combustion within the burner cavity is disclosed in representative U.S. Pat. Nos 3,820,320, 4,054,028, and 4,531,904. Alternately, at least a portion of the secondary air can be injected at the outlet of the burner so that the secondary combustion reactions occur largely in the enclosed combustion chamber or furnace which utilizes the heat produced by the burner. Such a method of NO.sub.x reduction is taught in U.S. Pat. Nos. 4,021,186, 4,245,980, and 4,488,869. Alternately, at least a portion of the secondary air and optionally tertiary air can be introduced directly into the furnace from ports in the furnace wall adjacent to the burner discharge so that the secondary combustion reactions occur completely within the furnace. This method of NO.sub.x reduction is disclosed in U.S. Pat. No. 4,629,413 and UK Patent Publication GB 2 048 456 A.
Staged combustion for NO.sub.x reduction in an air-fuel fired furnace can also be accomplished by the appropriate orientation and operation of multiple burners in the furnace. U.S. Pat. No. 4,403,941 discloses a method using burners oriented such that staged combustion zones are formed by primary burners with an air-fuel ratio of less than one, secondary burners downstream of the primary burners with an air-fuel ratio less than that of the primary burners, and air injection ports or afterburners downstream of the secondary burners.
U.S. Pat. No. 4,622,007 discloses a staged pre-mix burner in which fuel is combusted with oxygen or oxygen-enriched air in a first stage, and air is introduced in a second, downstream stage to complete the combustion process. Burner effluent then passes into a furnace. NO.sub.x is reduced by controlling and maximizing the amount of air introduced into the second, downstream stage relative to that introduced into the first stage. A post-mix burner and process in which fuel and oxygen or oxygen-enriched air are injected into a furnace in which the entire combustion process takes place are disclosed in U.S. Pat. Nos. 4,378,205 and 4,541,796. The injection of oxygen or oxygen-enriched air, through jets in the plane of the furnace wall surrounding a fuel injection jet also in the plane of the furnace wall, aspirates furnace gases into the oxygen jets before the oxygen mixes with the fuel, thereby lowering flame temperatures and thus reducing NO.sub.x formation.
U.S. Pat. No. 4,693,680 discloses a post-mix burner in which oxygen or oxygen-enriched air is injected through radially-spaced injection points located around a fuel injection point, all injection points being located in the plane of a furnace wall, wherein a smaller amount of oxidant is mixed with the fuel prior to injection into the furnace to stabilize the flame. By proper selection of the oxidant gas velocities, a stable flame is obtained. No discussion of NO.sub.x formation is presented in this patent.
U.S. Pat. No. 4,931,013 describes a burner which uses a lance protruding slightly beyond the burner face to inject pure oxygen at supersonic velocities into the flame. No discussion of NO.sub.x formation is presented in this patent.
Methods of injecting oxygen into rotary kilns to increase flame temperatures and improve kiln performance in the incineration of waste and the manufacture of cement and other materials are disclosed in U.S. Pat. Nos. 3,074,707, 3,441,634, 3,488,700, 4,927,357, and 5,000,102. In the method disclosed in each of these patents, a stream of oxygen is undershot or lanced indirectly into a the flame of an existing air-fuel burner to maximize heat transfer to the material in the kiln. The oxygen stream may also be directed toward the hot furnace charge and deflected therefrom indirectly into the flame. No discussion of NO.sub.x formation is presented in these patents.
Improved methods for reducing NO.sub.X emissions from furnaces, kilns, and other combustion equipment are required to meet increasingly stricter air quality regulations, particularly in oxygen-enriched combustion processes which can increase NO.sub.x formation over that of air-based combustion processes. Methods which modify existing combustion equipment without burner replacement will be particularly useful and economically attractive since is allows compliance without significant capital investment. The invention disclosed herein provides a simple, inexpensive, and effective method to reduce NO.sub.x formation in existing air-based or oxygen-enriched combustion processes, or alternatively is a method which is useful in the design and operation of new installations of such combustion processes.